It may consist, according to a well-known concept, in periodically burning the soot deposits.
This combustion sometimes occurs naturally when the temperature of the gases reaches by itself the level required for initiating oxidation of the particles. However, medium operating conditions generally lead to temperatures that are too low to spontaneously ignite combustion of the particles. This thus leads to clogging of the filter, which is a serious disadvantage for the engine efficiency and eventually imperils the operation thereof. Complete regeneration of the filter then has to be performed artificially and arbitrarily.
Many techniques have been developed to that end. They can be based on changes in the running of the engine: intake throttling, exhaust throttling, advanced injection lag, or connected with an energy supply in the exhaust gases or in the filter (electric resistor, burner, microwave . . . ). It is then necessary to control these various devices by means of an external control managed by a computer. Most often, the criterion taken into account for triggering regeneration is the back pressure in the exhaust line.
In order to facilitate regeneration of particle filters, a different and complementary approach of chemical nature consists in adding to the fuel an organometallic additive for example, that is thereafter found in the soot deposit, which generally leads to a decrease in the ignition temperature and therefore to an increase in the regeneration frequency.
Examples of the most commonly used additives are copper, iron, cerium, sodium . . . Studies show that, in the presence of such additives, partial regenerations can occur spontaneously for relatively low exhaust gas temperatures (.about.200.degree. C.).
However, for certain types of pipe, problems linked with the back pressure can remain, so that an exterior energy supply can be necessary, such as for example electric heating.
As regards energy consumption, most well-known systems using electric heating of the filter include global heating of the filtering element. This leads to a high energy consumption that is more or less controlled. Generally, the electric power required for triggering total regeneration of the filter is high and often hardly compatible with the electric resources on board the vehicle. Patent EP-B1-0,485,179 illustrates a system based on this principle.
French patent application EN.96/13,855 filed in the name of the applicant describes a concept allowing to adapt the electric power and the energy distribution in the filter to all the operating conditions of the vehicle. This concept consists in dividing the filtering unit into several zones, in equipping each of said zones with a resistor and, after detecting the global fouling of the filtering unit, in selectively activating one or more resistors. Valves are furthermore associated with the resistors in order to increase or to attenuate the effect of the resistors.
This regeneration modulation is however based on a general and global fouling evaluation.
Besides, the regeneration conditions can depend to a large extent on the state of fouling of the filter. In general, electric heating allows no flexible action according to the fouling of the filter.
The present invention allows this technology to be improved by furthermore achieving a local fouling evaluation.